a(n) = 2*a(n-1) + 4*a(n-2) - 6*a(n-3) - 3*a(n-4) for n > 3, with a(0)=1, a(1)=2, a(2)=4, a(3)=8.

Terms

a(0) =1a(1) =2a(2) =4a(3) =8a(4) =17a(5) =36a(6) =80a(7) =178a(8) =409a(9) =942a(10) =2212a(11) =5204a(12) =12377a(13) =29472a(14) =70592a(15) =169198a(16) =406801a(17) =978426a(18) =2357092a(19) =5679488a(20) =13696385a(21) =33032892a(22) =79703120a(23) =192321034a(24) =464168041a(25) =1120302822a(26) =2704242244a(27) =6527724428a(28) =15758096777a(29) =38040729336